1. Field of the Invention
The present invention relates to technology of a magnetic memory. More particularly, the present invention relates to the structure and access method for a magnetic memory cell and the circuit of a magnetic memory, wherein it at least has the properties of low writing current and using an adjacent memory cell as a reference memory cell.
2. Description of Related Art
Magnetic memory, for example, magnetic random access memory (MRAM), is also a non-volatile memory, which has such advantages as non-volatility, high density, high read/write rate, anti-radiation etc. Magnetic memory records data of logic “0” or logic “1” through the magnetoresistance produced by parallel or anti-parallel arrangement of the magnetic moments of the magnetic material adjacent to the tunnel barrier layer. Generally when data is written, a magnetic memory cell is selected by two current lines, for example, bit lines (BL) and write words line (WWL), by the intersection of the induction magnetic fields of the BL and the WWL, and the magnetoresistance of the selected memory cell is changed by changing the direction of the magnetization vector of the free layer. When the data is to be read, current flows through the selected magnetic memory cell so that the digital value of the stored data can be determined from the magnetoresistance being read.
FIG. 1 illustrates the basic structure of a magnetic memory cell. Referring to FIG. 1, cross current lines 100 and 102 with suitable current passing through are required to write a magnetic memory cell. The current lines 100 and 102 are also referred as, for example, bit line and word line based on the operation. When currents pass through the two conductive lines, magnetic fields of two directions will be produced to obtain the required magnetic fields to be supplied to the memory cell 104. The magnetic memory cell 104 is of stacked layer structure and includes a magnetic pinned layer which has a fixed magnetization vector or a total magnetic moment in a predetermined direction. Different magnetoresistances are produced for reading data by using the angle differences between the magnetization vectors of the magnetic free layer and the magnetic pinned layer. Besides, if the data is to be written, a writing magnetic field is to be supplied to switch the direction of the magnetization vector of the magnetic free layer. The data stored in the memory cell can be read through the output electrodes 106 and 108. The operation detail of the magnetic memory is well-known by those skilled in the art and therefore will not be described herein.
FIG. 2 illustrates the recording mechanism of a magnetic memory. Referring to FIG. 2, the magnetic pinned layer 104a has a fixed magnetic moment direction 107. The magnetic free layer 104c is disposed on the magnetic pinned layer 104a and between the layers there is a tunnel barrier layer 104b. The magnetic free layer 104c has a magnetic moment direction 108a or 108b. Since the magnetic moment direction 108a is parallel to the magnetic moment direction 107, the magnetoresistance being produced represents, for example, data of “0”. Contrarily, the magnetic moment direction 108b is anti-parallel to the magnetic moment direction 107, and the magnetoresistance being produced represents, for example, data of “1”.
The relationship between the resistance (R) and the applied magnetic field (H) of a magnetic memory cell is as illustrated in FIG. 3. The solid line represents the magnetic field curve corresponding to the magnetoresistance of a single magnetic memory cell. However, the magnetic memory device includes a plurality of memory cells, and the condition of each memory cell is different, thus, the magnetoresistance curve may have variations as shown by the dashed line, which may result in writing error. FIG. 4 illustrates the array structure of conventional memory cells. The left diagram in FIG. 4 illustrates an array structure, for example, for writing into the memory cell 140 through supplying magnetic fields Hx and Hy in two directions. The right diagram in FIG. 4 illustrates the action of the magnetic fields Hx and Hy to the memory cell 140. In the solid line area, the memory status of the memory cell 140 is not changed since the magnetic fields are small. While the magnetic fields in a limited area outside the solid line area are suitable for reversing the magnetic fields. Large magnetic fields are not suitable because the adjacent memory cells will be disturbed magnetically if the magnetic fields are too large. Thus, the magnetic fields in the operation area 144 are serving as operation window. However, since the other memory cell 142 will also experience the partial supplied magnetic fields, the partial supplied magnetic fields may also disturb the data stored in the other memory cell 142 because the operating conditions around the memory cell 142 are different. Thus, writing error may be caused to the single-layer magnetic free layer 104c in FIG. 2.
To resolve the foregoing problems, some improved technologies have been provided. FIG. 5 illustrates another design of a conventional magnetic memory cell. In FIG. 5, a stacked magnetic pinned layer is formed by stacking a pinned layer 160, a magnetic coupling spacer 162, and a reference layer 164. The pinned layer 160 has a magnetization vector 172 and the reference layer 164 has a magnetization vector 174, and the directions of the magnetization vectors are as shown in FIG. 5, which are perpendicular to the surface of the figure. In addition, the single-layer free layer 168 is disposed on the reference layer 164 and is spaced by a tunnel barrier layer 166. The direction of the magnetization vector 176 of the magnetic free layer 168 is as shown in FIG. 5, which may be leftward or rightward and is perpendicular to the magnetization vector 174. There is an electrode layer 170 on the magnetic free layer 168, and besides, there are an anti-ferromagnetic layer, a buffer layer, and another electrode layer (not shown) under the pinned layer.
The design of the magnetic memory cell in FIG. 5 is to make the coupling between the magnetization vectors 172 and 174 very weak, and the magnetization vector 174 can be changed by the externally supplied assisted field. When the magnetization vector 174 is rotated for certain angle by the assisted field, the direction of the magnetization vector 176 can be determined by the angle difference between the magnetization vectors 176 and 174, accordingly the binary data thereof can be determined.
FIG. 6 illustrates the magnetic memory circuit formed by the magnetic memory cell in FIG. 5. In FIG. 6, the magnetic memory cells are arranged in an array and are controlled by bit lines BLi, BLj and write word lines WWLi, WWLj. For example, when the data stored in the memory cell 180 is to be read, the magnetization vector 188 of the reference layer is rotated for certain angle, here the magnetization vector 186 of the free layer is in rightward direction, thus a lower magnetoresistance is produced. Here the adjacent memory cell 182 is used as the reference memory cell and a reference magnetoresistance is directly read. Since the reference magnetoresistance is in an intermediate state and the magnetization vectors 186 and 184 are perpendicular to each other. The data stored in the memory cell 180 can be determined after the magnetoresistances being compared by a sense amplifier (SA).
According to the conventional technologies as shown in FIG. 5 and FIG. 6, the magnetization vector of the magnetic reference layer is rotated by the assisted field. However, the magnetization vector of the free layer could be disturbed at the same time, as shown in FIG. 7. The magnetization vector 186′ of the free layer has departed from a magnetic easy axis of the free layer, and the magnetic field being supplied may cause change of status of the memory data. Thus, the conventional technologies described above may cause data errors.
Moreover, in U.S. Pat. No. 6,545,906, the single free layer is changed into a three layers structure and the magnetization vector of the free layer is rotated in toggle mode. However, since the data is stored by the direction magnetization vector of the free layer being parallel or anti-parallel to the direction of the magnetization vector of the pinned layer, the complex procedure of read before write is required and a set of reference cells is shared by a plurality of memory cells. The read before write process may cause at least slow operation rate, and the common reference bits may result in the reference memory cell being overloaded, which may affect the operation rate and also damages to the reference memory cell.
Accordingly, the manufacturers or designers are still devoting themselves to the searching of more satisfying magnetic memory cell and the design of driving method.